In recent years, the air/fuel ratio for an internal combustion engine, and particularly a gasoline engine, has been run lean to avoid unburned hydrocarbons in the exhaust. This is not so much required for fuel economy, but to minimize the unburned hydrocarbons discharged from the exhaust into the atmosphere. In some jurisdictions, it has been found that, even with lean running of the engine, the exhaust products included a significant amount of hydrocarbons. To overcome this, after-burning catalytic converters have been applied.
In other jurisdictions, after-burning catalytic converters have not been used because such converters require that the engines run on unleaded fuel in order to avoid poisoning of the catalytic converter. Thus, an entire new fuel supply system must be created in order to achieve after-burning with catalytic converters.
It has been found that a hot exhaust manifold, which runs hotter in today's lean running engines, causes formation of NO.sub.x in the ambient air around the exterior of the manifold. There is still need both to reduce the unburned hydrocarbons in the exhaust gases of gasoline engines and to reduce the passive formation of NO.sub.x in the ambient air surrounding the hot exhaust manifold.